Don't double-escape SendDigits; Compatibility with Rails 3.x.x

When I included a pound sign in send_digits, like so:

``` ruby
Twilio::Call.create(
  to: "...",
  from: "...",
  url: "...",
  send_digits: "34#"
)
```
I started encountering [this Twilio error]( https://www.twilio.com/docs/errors/21206), with the following message:

> Invalid sendDigits: 34%23

After much digging, I discovered that the problem was that Twilio::Call.create escapes `:send_digits` with CGI.escape. This is unnecessary because HTTParty already does it. So, this caused `:send_digits` to be escaped twice, making the string invalid.

This pull request contains a fix for that, and also a minor tweak to the gemspec to make the gem compatible with Rails 3.x.x. These versions of rails require builder 3.0.0, so the requirement of builder 3.1 was too high.